Density Functional Theory Calculation And Experimental Research On Manganese Oxides As ORR Electrocatalyst

Electrochemical oxygen reduction reaction (ORR) is the main limitation in fuel cells and metal-air batteries due to its sluggish kinetic properties. Exploration of low price and high active electrocatalysts is the key to resolve the issue. Recently, manganese oxides (such as MnO2) attract more and more interests for the replacement of Pt due to their low price, rich in sources and good stability. The main drawback of MnO2is the low activity, which is still far away from that of Pt. Many strategies have been developed to enhance the activity of MnO2, such as nanostructuring, doping metal ions and complexing with carbons. However, the common modification methods are either costly or complicated. Therefore, developing facile, novel and efficient strategy to improve the activity becomes more and more urgent. In the thesis, by combining both density functional method (DFT) calculations and experiments, the influence of oxygen-vacancy and hydrogenation has been used to easily enhance the activity of β-MnO2. Moreover, the effect of β-MnO2on the activity of Pt cluster has also been investigated. The main contents are summarized below.1. By employing DFT calculation, the oxygen-vacancy (110) surface of hydroxyl β-MnO2was constructed. Mn atoms with partial reduced valence were exposed on oxygen-vacancy surface. Comparing with the perfect hydroxyl β-MnO2(110) surface, the oxygen-vacancies could enhance the adsorption of O2and HO2, reducing the energy barriers of thermoldynamic and kinetics in ORR, and hence exhibit better catalytic properties. Different concentrations of oxygen vacancies (none,0.16/ML (monolayer) and0.33/ML of vacancies) were investigated, and the results showed that0.16/ML (monolayer) of oxygen vacancies demonstrated the best kinetic properties. These results indicate that inducing oxygen vacancy is an efficient method to improve the activity of β-MnO2.2. Using DFT calculation, a new structure of hydrogenated β-MnO2was constructed. The cell lattice was diordered and the eletronic structure became half-metallic after hydrogenation. The surface H could improve the adsorption of O2through hydrogen bonds and hence enhance the ORR activity. In experiments, the hydrogenated β-MnO2nanorods were successfully prepared though a facile and moderate hydrogen treatment. Comparing with pristine β-MnO2nanorods, the morphology of the hydrogenated β-MnO2was sustained while the microstructure was changed. The electrochemical measurements demonstrated that β-MnO2nanorods with hydrogenation had larger limited current, more positve half-wave potential, more electron transfer numbers and better stability. In Li-air batteries, the capacity using hydrogenated β-MnO2nanorods as cathodic catalysts showed～6000mAh g-1carbon with a resanoable capacity retention of20cycles.3. Using DFT calculation, Pt8cluster complex loaded on β-MnO2was constructed. In the composite, Pt atoms gave out electrons to β-MnO2, exhibiting positive charged state and the shift away of the d-band center from the Fermi level. Comparing with Pt8cluster, the adsorption of OH radicals was weakening on the complex, which accelerated the kinetics of ORR on Pt. This research demonstrated an atomic-scaled investigation of metal-metal oxides interaction with the help to better understanding the ORR mechanism.